1. Field of the Invention
The present invention relates to an optical functional device and a fabrication method thereof, more particularly, an optical functional device based on a Mach-Zehnder interferometer and a fabrication method of such an optical functional device.
2. Description of the Background Art
Attention is now focused on the wavelength division multiplex (WDM) technique to improve the transmission capacity of optical fibers. The usage of WDM technique allows the transmission ability of an optical fiber already installed to burst up to more than several ten times. The wavelength converter is known as one optical functional device employed in the light wave network of multiple wavelength. The wavelength converter is an important optical device indispensable in the WDM optical communication technique.
The wavelength converter converts the wavelength of input signal light into another wavelength differing from that of the input signal light. Wavelength conversion allows wavelength routing to be conducted at another site in the light wave network. Accordingly, all-optical network architecture in the future will become possible.
A conventional wavelength converter is based on, for example, a Mach-Zehnder interferometer, providing output signal light of a wavelength differing from that of the input signal light through constant light. F. Ratovelomanana et al. present “Regeneration improvement in all-optical wavelength converter, based on a Mach-Zehnder interferometer, by means of phase-shifter section”, Electronics Letters, The Institution of Electrical Engineers (IEE), Sep. 11, 1997, Vol. 33, No. 19, pp. 1629-1630. A conventional wavelength converter disclosed therein includes a phase control element in addition to a semiconductor optical amplifier (SOA) that amplifies constant light and modifies the refractive index. The phase control element is directed to adjust the phase difference of constant light split to two arms of a Mach-Zehnder interferometer. The provision of a phase control element in addition to a semiconductor optical amplifier allows the extinction ratio of the output signal light to be improved.
Some conventional wavelength converters (optical functional device) have a configuration in which the input signal light as well as the constant light passes through the phase control element of the Mach-Zehnder interferometer. In this case, there was the disadvantage of the intensity of the input signal light being attenuated by the phase control element depending on where the signal light is input.